Melting magnesium-base alloy



Nv. 1l, 1941.

F. c. BENNETT ET AL MELTING MAGNESIUM-BASE ALLOY Filed Aug. 30, 1940 [lm er Surface Carbu'l'zed Fam ace Cast Meel Melz Pot Patented Nov. 11, 1941 MELTING MAGNEswM-BASE ALLOY Foster C. Bennett and Richard I. Thrune, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Application August 30, 1tl40, Serial No. 354,788

(Cl. 'l5-67) 6 Claims.

This invention relates to a method and apparatus for melting magnesium-base alloys.

Inthe melting of magnesium-base alloys, the art has been limited largely to the use of vessels formed of ordinary low-carbon low-alloy steel.

vThus castfiron containers are too porous to the metal and the fluxes used. while the more common special4 alloy and non-metallic vessels are frequently chemically reactive with the molten metal or else are -too fragile or tooexpensive to be commercially feasible. However, steel containers are themselves subject tothe disadvantage that during prolonged use they are attacked somewhat by the molten alloys, particularly those alloys containing a minor proportion of aluminumy or manganese. This corrosion, while significant even in ordinary meltingpractice, is lparticularly serious in settling treatments and in those alloying processes in which aluminum or" manganese is incorporated into a mass of molten magnesium. Such corrosive attack is unfortunate not only in lthat it results in the weakening and eventual failure of the vessels, but also in that it frequently leads to contamination of the alloy by traces of iron. v

Efforts are commonly made to minimize these eifects by surrounding the body of molten alloy with a film of protective ilux which is intended to prevent the alloy from touching the-walls of its container. However, these efforts have been f largely unsuccessful, since during stirring, ladling,

and pouring it is a virtual impossibilityjfto maintain the flux film intact. aridthe alloy itself invariably is in direct contact with the vessel during an appreciable proportion of the time. As a result, the corrosive attack on steel containers by magnesium-base alloys has until now remained a serious practical problem.

The principal object of the present invention, then, is to overcome these existing dilculties by providing a method of treating steel vessels to render them more resistant to corrosion by magnesium-base alloys, especially those alloys containing a minor proportion of at least one of the metals aluminum and manganese.

According to the invention, this object is attained by carburiaing the inner surfaces of the steel vessel prior to the introduction of the magnesium-base alloy. The carburized coating formed eifectively retards corrosion of the vessel by the molten alloys without in any way sacrilicing its mechanical strength. In addition, in contrast to previous treatments, the protective layer may be easily produced with the simplest of equipment and without the use of expensive reagents.

Carburization according tothe invention may be applied to vessels formed of any of the steels, including alloy steels, commonly employed as containers for molten magnesium-base alloys. However, as a matter of economy, vessels of ordinary low-carbon low-alloy steel are usually selected, and, when carburized, attain a corrosion resistance entirely adequate for almost all purposes.

InY general, the treatment may advantageously be carried out on steel vessels used as containers for molten magnesium-base alloys in al1 the operations to which such-alloys are subjected. As noted, the process is particularly useful in retarding the corrosion of the direct-fired caststeel pots used in melting and settling operations, and in alloying vprocesses involving magnesiumbase alloys containing a minor proportion of at least one of the elements aluminum and manganese. These latter operations are carried out by heating a body of molten magnesium under a protective iiux, and incorporating the aluminum or manganese therein. Fior all these purposes, the carburized pots are used in a manner entirely similar to `that of .thesteel pots used heretofore.

In so far as it has been possible to determine,

any of the carburizing treatments ordinarily ap,-y

plied to steel is operable in producing a surface resistant to attack by magnesium-base alloys. Thus gas and llame treatments, liquid and fsolid carburizations, cyanidation, andthe like are all satisfactory, being carried out in accordance with known practice. In general, it is customary to carburize the entire inner surface o1v the steel vessel, although the process may if desired be limited to those portions of the vessel likely to come into contact with the molten alloy or alloying constituents. The treatment is carried out for a time suilicient 'to produce a carburized layer having substantial thickness and a signif icantly greater carbon content than the base metal. Instead of using direct carburizationv as described, the carburized layer may also be prom vided by casting in place or otherwise inserting a lining of carburized (i. e. highwarbonlsteel or iron over the inner surfaces oi' the steel vessel.

In the case of cast-steel melting pots, the use of carbon pastes provides a particularly convcnient method of effecting carburization. Thus, the inner surfaces of the pot are first coated with a layer of paste consisting essentially of finely divided carbon or graphite and a heat-hardening binder therefor, such as a heavy liquid tar. (Such pastes are described in detail in application Serial No. 312,550, filed January 5, 1940, by Richard I.

'melting pot having a carbon content of roughly 0.1 percent, to produce a carburized layer having a thickness as much as 0.25 to 1.0 inch or more and a carbon content of 0.7 to 1.2 per cent, or even higher, if desired. When suicient carburization has been effected, the lining is removed from the pot, after which the pot is ready for use according to the invention.

The accompanying self-explanatory `diagrammatic drawing will further serve to illustrate the invention. A

It is tobe understood that the foregoing description is illustrative rather than strictly limitative, and that the invention is oo -extensive in scope with the following claims.

We claim:

1. In a process wherein a magnesium-base alloy is maintained in the molten state in a steel vessel, the improvement which consists in retarding corrosion of the vessel by the molten alloy and reducing contamination of the alloy by heating the alloy in such steel vessel, the inner Surfaces of which exposedto contact with the molten alloy have previously been provided with a carburized iron layer.

2. In a process wherein a magnesium-base alloy containing a-minor proportion of atleast one of the metals aluminum and manganese is melted in a cast-steel pot, the improvement which consists in retarding corrosion of the pot by the molten alloy and reducing contamination of the alloy by melting the alloy in such cast-steel pot,

aacaaao the inner surfaces of which have been previously carburized.

` 3. In a process for forming a magnesium-base alloy wherein at least one of the alloying metals aluminum and maganese is'incorporated into a vmass of-molten magnesium maintained in a lowcarbon low-alloy cast steel pot, the improvement which consists in heating the molten magnesium under a protective ux and incorporating the alloying metal therein in such cast-steel pot, the inner surfaces of which have previously been carburized.

4. In a furnace for melting magnesium-base alloys containing a minor proportion of at least one of the metals aluminum and manganese, an. improved melting pot consisting o a low-carbon low-alloy cast-steel pot the inner surfaces of y which have been carburized to render the same more resistant to corrosive attack by the molten alloys.

5. In a process wherein a magnesium-base molten alloy and reducing contamination of the alloy by carburizing the inner surfaces of the pot prior to melting the alloy therein, said carburizing operation comprising coating the inner surfaces of the pot with a paste consisting essentially of finely-divided carbon and a heathardening binder therefor, baking to set the binder, then heating the resulting carbon-lined pot for a time sufficient to produce substantial carburization of the inner steel surface thereof,

and thereafter removing the lining. l

FOSTER C. BENNETT..

RICHARD I, THRU'NE. 

